I have tended to do this under the image in question. Say I was using a CC-BY image that I needed to attribute to someone else, I would have a source/copyright note & attribution in small text under the image on the slide. In the case you mention of using a CC-BY-SA image in the CC-BY work, I would just include the letters CC-BY-SA or a small version of the CC-BY-SA icon alongside my attribution to the author of the work.
I would caution against a separate slide never shown; you need to attribute the image to the originator of the work at or near to your usage of it. If, as you should, be indicating the source when you show the image, it is trivial to add CC-BY-SA or the CC icon to indicate the different terms.
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